Heavy metal pollutants are different from other organic pollutants. Many organic pollutants can be reduced or eliminated through physical, chemical or biological purification of the natural world. Heavy metal pollutants are difficult to be degraded in the environment, causing great harms to the ecological environment and human health and having potential environmental risks. Industrial and agricultural activities generate a large amount of wastes containing heavy metals. The heavy metals enter rivers, lakes and other bodies of water through processes such as migration and release, and most of them are rapidly transferred to sediments and suspended matters. Suspended matters will also gradually become part of the sediments as they are transported by the water stream. The content of heavy metals in sediments is usually much higher than that in water bodies and will gradually be released as the main endogenous source of metal pollutions in water bodies with the changes of environmental conditions. In addition, the distribution of heavy metal content in water is irregular, but it shows a clear horizontal distribution in sediments. The study of heavy metals in sediments can help to trace pollution sources and to understand the diffusion and potential environmental risks of heavy metals. The sediment area is a place where fish, shrimp, shellfish and other benthic animals carry out metabolism. Through the food chain, heavy metals in sediments can be enriched ten thousand times in benthic animals, which is very harmful to aquatic environment ecosystem. Therefore, it is of great significance to study the ecological risk of heavy metals in sediments.
The ecological risk of heavy metals in sediments depends not only on their content, but also on their release from sediments into water bodies, and more directly on their toxic effects on organisms. In particular, when assessing the ecological risk of multiple heavy metals, heavy metals' toxic coefficients need to be introduced due to the different toxic effects of each heavy metal. Moreover, the toxic effects are also affected by biological species, and the biological species are differently distributed in different watersheds.
At present, the ecological risk assessment of heavy metals in sediments is usually evaluated for single metal, such as the method of enrichment factor and geoaccumulation index. However, there are few comprehensive evaluations for multiple metals, such as Excess after Regression Analysis method. Most of these methods do not consider the biotoxicity of heavy metals. Only the potential ecological risk index (RI) proposed by a Swedish scientist, Hakanson, in 1980, comprehensively evaluates the ecological risk of various heavy metals and considers the biotoxicity of different heavy metals on organisms. However, due to the lack of toxicity data of heavy metals at that time, Hakanson calculated the toxicity coefficients of heavy metals indirectly according to the abundance of heavy metals in the earth's crust. He did not calculate the toxicity coefficients of heavy metals on organisms directly, nor did he take into account the sensitivity of different species to heavy metals.
At present, Hakanson's method is still the main method to assess the ecological risk of heavy metals in the field of environmental science and engineering. Patent application CN201410137454.3 discloses “a method for determining ecological risk of heavy metal pollutions in river and lake sediments”. This patent application mainly uses the principle of statistics and sampling calculation to determine the distribution of various heavy metals, in which the toxicity coefficient of heavy metal pollutants is still based on the calculation of its crustal abundance. Patent application CN201410074703.9 discloses “a method for comprehensive ecological risk assessment of heavy metal pollutions in three phase space of rivers”. This patent application also uses Hakanson's method directly to calculate the toxicity coefficient of heavy metals.
The method proposed by Hakanson is based on the abundance of heavy metals in the earth's crust to calculate the toxicity coefficient of heavy metals, which is a compromise due to the lack of toxicity data for heavy metals at that time. In addition, the existing methods for ecological risk assessment of heavy metals in sediments still lack consideration of the toxic effects of heavy metals on aquatic organisms, and do not take into account the differences in the distribution of biota in different watersheds.
Today, as the toxicity data of heavy metals is more and more complete, the assessment of the ecological risk of heavy metals in sediments urgently requires a more accurate method for calculating the toxicity coefficients of heavy metals in order to evaluate the ecological risk of heavy metals in sediments more accurately.